Examination of the Martian atmosphere by NASA's Curiosity Mars rover confirms that some meteorites that have dropped to Earth really are from the Red Planet.

A key new measurement of the inert gas argon in Mars' atmosphere by Curiosity's laboratory provides the most definitive evidence yet of the origin of Mars meteorites while at the same time providing a way to rule out Martian origin of other meteorites.

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The new measurement is a high-precision count of two forms of argon -- argon-36 and argon-38 -- accomplished by the Sample Analysis at Mars (SAM) instrument inside the rover. These lighter and heavier forms, or isotopes, of argon exist naturally throughout the solar system. On Mars the ratio of light to heavy argon is skewed because much of that planet's original atmosphere was lost to space. The lighter form of argon was taken away more readily because it rises to the top of the atmosphere more easily and requires less energy to escape. That left the Martian atmosphere relatively enriched in the heavier isotope, argon-38.

Years of past analyses by Earth-bound scientists of gas bubbles trapped inside Martian meteorites had already narrowed the Martian argon ratio to between 3.6 and 4.5 (that is 3.6 to 4.5 atoms of argon-36 to every one of argon-38). Measurements by NASA's Viking landers in the 1970s put the Martian atmospheric ratio in the range of four to seven. The new SAM direct measurement on Mars now pins down the correct argon ratio at 4.2.

"We really nailed it," said Sushil Atreya of the University of Michigan, Ann Arbor, lead author of an Oct. 16 paper reporting the finding in Geophysical Research Letters. "This direct reading from Mars settles the case with all Martian meteorites."

One reason scientists have been so interested in the argon ratio in Martian meteorites is that it was -- before Curiosity -- the best measure of how much atmosphere Mars has lost since the planet's wetter, warmer days billions of years ago. Figuring out the planet's atmospheric loss would enable scientists to better understand how Mars transformed from a once water-rich planet, more like our own, into today's drier, colder and less-hospitable world.

Had Mars held onto all of its atmosphere and its original argon, its ratio of the gas would be the same as that of the sun and Jupiter. Those bodies have so much gravity that isotopes can't preferentially escape, so their argon ratio -- which is 5.5 -- represents that of the primordial solar system.